Field of the Invention
The present invention relates to a magnetic-disk substrate, a magnetic disk, and a magnetic-disk drive device.
Background Information
Nowadays, in order to record data, hard disk drives (HDDs) are incorporated in personal computers, DVD (digital versatile disc) recording devices, or the like. In particular, a magnetic disk obtained by providing a magnetic recording layer on a substrate is used in a hard disk drive used in a device for which portability is a prerequisite, such as a laptop personal computer, and magnetic recording information is recorded in or read from the magnetic recording layer with a magnetic head that flies slightly away from the surface of the magnetic disk. A glass substrate is preferably used as a magnetic-disk substrate due to the fact that it has a property of being unlikely to undergo plastic deformation compared to a metal substrate (an aluminum alloy substrate) or the like.
Also, in order to meet the demand for an increase in the storage capacity of hard disk drives, attempts have been made to increase the magnetic recording density. For example, the magnetic recording information area (the recording bit) has been made smaller using a perpendicular magnetic recording method in which the direction of magnetization in the magnetic recording layer is perpendicular to the surface of the substrate. In a perpendicular magnetic recording magnetic disk, an adherent layer, a soft under layer (SUL), a base layer, a magnetic recording layer, a protecting layer, a lubricant layer, and the like are formed on a metal substrate or a glass substrate in the stated order, for example. The storage capacity of one disk substrate can be increased by adopting a perpendicular magnetic recording system. Furthermore, in order to further increase the storage capacity, the distance from the magnetic recording layer is made extremely short by allowing only a recording and reproduction element portion of the magnetic head to further protrude from the slider surface toward the magnetic disk, thus further improving the accuracy of the recording and reproduction of information (improving the S/N ratio). Note that the control of the recording and reproduction element portion of the magnetic head is called a DFH (dynamic flying height) control mechanism, and a magnetic head provided with this control mechanism is called a DFH head. In order to avoid collision and contact with the magnetic head or the recording and reproduction element portion that further protrudes therefrom, a magnetic-disk glass substrate used in an HDD in combination with this DFH head is produced such that the surface unevenness of the substrate is as small as possible.
The DFH control mechanism is a mechanism that performs control so as to make the distance from the head surface to the surface of the magnetic disk close to several nanometers by controlling the expansion of a reading element and writing element (collectively referred to as a “R/W element portion” hereinafter) of the head through providing a heater coil around the R/W element portion and controlling electric current that flows through the heater coil. In this case, in order to stably perform writing to or reading from the magnetic disk for a long period, it is not preferable that the surface of the magnetic disk comes into contact with the R/W element portion and the R/W element portion undergoes abrasion. Therefore, it is preferable that the surface of the magnetic disk is a smooth surface having small waviness, and in particular, the waviness of the glass substrate, which is a substrate for the magnetic disk, needs to be extremely small.
Under such a circumstance, a glass substrate for a magnetic recording medium in which a change amount of the microwaviness of the entire surface of a recording and reproduction region of the glass substrate for a magnetic recording medium is controlled in a predetermined range is known (Japanese Patent No. 4977795). Specifically, the main flat surface of the glass substrate for a magnetic recording medium is such that upon measuring the microwavinesses (nWq) of grid-like evaluation regions that are set on the entire main flat surface including the entire surface of a region that serves as the recording and reproduction region when the glass substrate is used to prepare a magnetic disk, a percentage of an absolute value (ΔnWq) of a change amount of microwavinesses between one evaluation region and an evaluation region adjacent thereto, relative to the microwaviness of the one evaluation region (change percentage) is 10% or less. Also, an average value of the microwavinesses (nWq) measured in these grid-like evaluation regions is 0.080 nm or less. The microwaviness has a period (a wavelength) of 0.2 μm to 1.8 μm.
However, when a magnetic layer was formed on the above-described glass substrate to produce a magnetic disk and the characteristics of the DFH head were evaluated, an abrasion mark was confirmed on the tip of the R/W element portion. This was caused by the R/W element portion coming into contact with the surface of the magnetic disk and being rubbed.